This invention relates to a fuel container, and more particularly to a fuel container of polymeric material preferably made by blow molding.
In the automotive industry polymeric fuel containers are being used because they can be manufactured in a cost-efficient manner, are of high mechanical stability and readily deformable in accidents, and greatly inhibit permeation of hydrocarbons. The best results as to the overall qualities of the polymeric fuel containers have been obtained by a six-layer so-called COEX-structure. This is a multi-layer system manufactured in a single process (co-extrusion) and including two layers of high density polyethylene (HDPE) which enclose a barrier-layer of an ethylene vinyl alcohol copolymer (EVOH) and a layer of treated, recycled orxe2x80x9cre-grindxe2x80x9d plastic material. The EVOH-layer, which is not directly connectable to the HDPE, has an adhesive layer provided on both of its sides for connection to the adjacent layers so that the total structure comprises six layers. The layer of recycled or re-grind material is of a thickness which is about 35 to 45% of the total thickness of the fuel container wall and consists of a mixture of scrap materials resulting from the manufacturing of containers, i.e. it is both of HDPE and of EVOH. While HDPE is cheap and has good mechanical characteristics, it is a poor barrier against permeation of hydrocarbons. This is why the relatively thin EVOH-layer is used, which while being expensive, is an excellent barrier against permeation of hydrocarbons.
Presently, the State of California generally has the most stringent requirements for the reduction of total vehicle hydrocarbon emissions. As a general rule the other states in the U.S. and many other countries will adopt the Californian regulations after some time. Under the provisions of such future regulations the level of total vehicle hydrocarbon permeation must not exceed 0.5 g per day. To achieve this level, it has been estimated that the hydrocarbon emissions from the vehicle fuel system must not be more than 150 mg per day, which would result in a static permeation of less than 55 mg per day when production and durability parameters are considered. However, the fuel container is only a part of the total fuel system, and further estimates have shown that permeation through the container wall should not exceed 5 mg per day in order to meet the above requirements. The above described typical six-layer COEX-structure, however, only provides permeation levels of about 20 mg or less per day. One possibility to improve the performance of the six-layer COEX-structure would be to increase the thickness of the EVOH-layer from about 150 xcex7m to about 1.0 mm. Apart from substantially increased costs this would cause production problems and deteriorate the mechanical properties of the fuel container because EVOH has relatively poor impact resistance.
According to the present invention, a wall of a fuel container is formed of at least two wall shells which are separated from each other at least partially by an air gap. The dual wall structure helps to substantially improve the mechanical properties of the fuel container. Preferably, each wall shell consists of a plurality of layers of different polymeric or plastic materials. In a preferred embodiment of the invention a four layer co-extruded structure is provided; each wall shell comprises a layer of high density polyethylene (HDPE), a layer of filler material of treated, recycled plastics material referred to asxe2x80x9cre-grindxe2x80x9d and a vapor barrier layer of a plastic material substantially impermeable by hydrocarbons. The HDPE-layer may be one surface layer of each wall shell while, in contrast to prior art six-layer coextruded structures, the vapor barrier layer is preferably the other surface layer of each wall shell. Depending on the material properties of the vapor barrier layer, an additional adhesive layer may also be necessary for connection with the adjacent layer.
In a preferred embodiment of the invention, the vapor barrier layer of the inner wall shell forms the innermost layer of the fuel container and the vapor barrier layer of the outer wall shell forms the outermost layer of the fuel container.
In order to facilitate integration of further components of the fuel system into the fuel container, such as a fuel pump and fuel level indicator, it is desirable to form the container from two separately made wall halves welded together. If the vapor barrier layer of the inner wall shell faces the interior of the container, welding the two wall halves together will advantageously result in an integral, continuous vapor barrier layer so as to avoid a so-called permeation window. A preferred material of the vapor barrier layer is an ethylene vinyl alcohol copolymer (EVOH).
The total hydrocarbon emission can be substantially reduced by communicating the air gap between the wall shells to a suction discharge device for hydrocarbon vapors, which preferably includes an activated carbon filter and which feeds the hydrocarbon vapors either to the combustion chamber in the engine of the automotive vehicle or back into the interior of the container. A further advantage of the dual wall structure of the invention is improved thermal and acoustic insulation of the fuel within the container from the ambient as compared to prior art single-wall fuel containers. This also assists in reducing the permeation rates through the fuel container walls. A further improvement in this respect, which additionally improves the mechanical stability of the fuel container, is obtained when the air gap between the wall shells is filled at least partially by a foam of a preferably open cell plastic material.
To allow for assembly of fuel conduits and the like, the fuel container is provided with at least one container opening extending axially through the wall shells. For sealing purposes a substantially tubular duct element extending through the container opening can be used. The duct element is preferably formed of the same material as the vapor barrier layer. Because in the preferred embodiment the vapor barrier layer forms the innermost and outermost layers of the fuel container, the duct element may be readily welded to at least one of the barrier layers.